1. Field of the Invention
The present invention relates to an image forming apparatus and a process cartridge.
2. Discussion of the Background
As an image forming apparatus including a charging device to uniformly charge the surface of an image bearing member by applying a charging bias in which an AC voltage is overlapped with a DC voltage to a charging member provided facing the surface of the image bearing member, there is known an image forming apparatus in which a charging member such as a charging roller formed of a material having an electric resistance to which a charging bias is applied is provided in contact with or in the vicinity of the surface of the image bearing member.
As a system of applying a charging bias to a charging member, there are known a DC application system which applies a charging bias only composed of DC voltage component and an AC application system in which an AC voltage is overlapped with a DC voltage for application of a charging bias. In either of the systems, the optimal charging bias required to obtain a desired surface voltage for the image bearing member varies depending on changes in the charging conditions, such as environmental change such as temperature and humidity, and change in the contact state or the distance (gap) between the surface of an image bearing member and the charging member. For example, when the temperature of a charging member falls, the resistance thereof increases, and when the temperature of a charging member rises, the resistance thereof decreases. Therefore, in a low temperature environment, since the resistance of the charging member is high, discharging required for charging treatment tends to hardly occur. Thus, the surface of the image bearing member is not charged up to the target charging voltage, which tends to lead to insufficient charging.
By contrast, in a high temperature environment, since the resistance of the charging member is low, excessive discharging tends to occur. This easily leads to deterioration of the surface of the image bearing member or the occurrence of filming in which toner or external additives thereto fixates on the surface of the image bearing member. Therefore, as the temperature changes, it is desirable that the DC voltage of the charging bias be adjusted in the DC application system and the AC voltage (Vpp: peak to peak voltage) of the charging bias be adjusted in the AC application system such that the charging bias applied to the charging member is optimized for the temperature at the time.
In addition, the amount of discharging in the AC application system is relatively large in comparison with that in the DC application system. Therefore, it is desirable that the AC voltage of the charging bias be as low as possible to avoid the occurrence of filming. At the same time, however, an AC voltage of the charging bias that is too low tends to lead to insufficient discharging, resulting in the occurrence of bad charging. Therefore, when the temperature rises and the resistance of the charging member declines, the AC voltage is reduced to avoid excessive discharging. On the other hand, when the temperature falls, the AC voltage is increased. Thereby, the AC voltage of the charging bias is controlled to maintain the optimal value.
Unexamined published Japanese patent application No. 2001-201921 describes a charging bias control method for the AC application system. In this control method, the AC electric current when the peak to peak voltage Vpp less than twice Vth is applied to at one or more points of the image bearing member and the AC electric current when the peak to peak voltage Vpp less than twice Vth is applied to at two or more points of the image bearing member are measured, where Vth represents the discharging starting voltage to an image bearing member when a DC voltage is applied to a charging member. Then, based on these measured values, the peak to peak voltage Vpp of the AC voltage to be applied to a charging member is adjusted at the next image formation.
According to the description in unexamined published Japanese patent application No. 2001-201921, it is possible to maintain the optimal AC voltage which secures sufficient discharging without causing excessive discharging even when the resistance of the charging member changes according to environmental changes such as a temperature change.
In addition to the method described in unexamined published Japanese patent application No. 2001-201921, there is another method of adjusting a charging bias which can maintain the most suitable voltage at which sufficient discharging is secured without causing excessive discharging even when the resistance of the charging member changes according to the environmental change such as a temperature change. Specifically, the method involves controlling a constant electric current such that the AC electric current (effective value) flowing through a charging member matches a target value (charging electric current target). When the temperature rises and the resistance of the charging member declines, the electric current flowing through the charging member surpasses the target value, and accordingly, the AC electric current is controlled to decrease. When the temperature falls and the resistance of the charging member increases, the electric current flowing through the charging member falls below the target value, in which, the AC electric current is controlled to increase. Therefore, when the resistance of the charging member varies according to the environment change such as temperature change, it is possible to maintain the optimal AC voltage which secures sufficient discharging without causing excessive discharging.
On the other hand, there is an image forming apparatus which forms images on various kinds of recording media with different definitions by switching the image formation speed. When the image formation speed is different, the surface travel speed of an image bearing member is different, and naturally, the time to be taken for the surface portion on the image bearing member per unit area to pass through the charging area of the charging device varies. Therefore, when an AC application system is adopted and the image formation speed is high, it is known that the AC voltage frequency of the charging bias is short, which causes striped uneven density according to the frequency. In addition, when an AC application system is adopted and the image formation speed is low, it is also known that the AC voltage frequency of the charging bias is high, which easily causes filming on the surface of the image bearing member. Therefore, in an image forming apparatus which forms images by switching image formation speeds, it is desired to change the AC voltage frequency of the charging bias to a frequency suitable for the image formation speed every time the image formation speed is switched.
In general, when images are continuously formed by an image forming apparatus, the temperature therein rises. When there is a long interval between successive image formations, the temperature in the image forming apparatus declines, meaning that the temperature changes second by second. Since the optimal AC voltage varies according to the changes in the temperature in the image forming apparatus, it is desirable to increase the frequency of charging bias adjustment. However, depending on the status of use of an image forming apparatus, increasing the frequency of charging bias adjustment may result in significant extension of the waiting time for a user, for the reason described below.
Specifically, when a large number of images are continuously formed, it is desirable to adjust the charging bias in the middle of the continuous image formation. In a typical image forming apparatus which can switch image formation speeds, the charging bias is adjusted to maintain the optimal AC voltage by a single image formation speed (a particular image formation speed). Therefore, for example, a user who continuously forms images in a large number at a speed different from the particular image formation speed changes the image formation speed for charging bias adjustment during the particular image formation speed on every occasion, resulting in extension of the time to be taken for adjusting the charging bias. Images are not formed while the charging bias is adjusted, and thus, the time to be taken for adjusting the charging bias is tantamount to waiting time for the user. Therefore, in this case, the time to be taken for changing the image formation speed results in an increase in the waiting time for the user.